1. Field of the Invention
This invention pertains generally to III-V semiconductor devices (so called because one element is obtained from column III of the Periodic Table of Elements and the other from column V), and more particularly to transferred electron III-V semiconductor photocathode construction.
2. Description of the Prior Art
Semiconductor photocathodes are used in various light sensing applications. In a typical transmission photocathode, the backside of the photocathode emits electrons into a vacuum in response to photons (visible and infra-red light) incident on the front side of the photocathode. The efficiency of this production is the photocathode's quantum efficiency measure. In a simple diode device, the electrons that emit from, or escape, the surface of the photocathode into the vacuum are accelerated by an electric field and are attracted to and strike a phosphor target screen. The phosphor emits light in response to the incident electrons which may be of a different wavelength than the light incident on the photocathode.
Phonton absorption causes the electrons in the valence band of a photon absorbing layer of the photocathode to elevate to the lower valley (gamma valley) of the conduction band. The most efficient photocathodes used in modern light sensing and imaging applications are so called Negative Electron Affinity (NEA) photocathodes which rely on gamma valley transport of electrons almost exclusively.
Although NEA photocathodes have excellent sensitivities, their long wavelength response is limited to about 1000 nm by greatly reduced electron surface escape probabilities for semiconductors with bandgaps smaller than about 1.2 eV (wavelengths longer than 1000 nm). Work function and surface barrier effects at the vacuum-semiconductor interface limit the successful transport of photoexcited electrons into vacuum. In order to overcome the surface barrier effects in long wavelength photocathodes, various externally biased photocathodes have been studied over the years. Externally biased photocathodes can, in principle, extend the long wavelength cutoff by lowering the vacuum energy level relative to the Fermi level in the bulk photon-absorbing active layer. A number of p-n junction, MOS, field-emission, and heterojunction bias-assisted photocathodes have been proposed and experimentally studied, but none prior to the development of the transferred electron (TE) photocathode patented by Bell, U.S. Pat. No. 3,958,143 ('143), has shown reasonably efficient photoemission combined with the required low dark current emission to be of practical interest. A complete description of the principles of operation of the TE photocathode, together with a discussion of the limitations of NEA photocathodes, is found in Bell '143. The present invention belongs to the class of TE photocathodes.
In 1974, Bell, et al. demonstrated a bias-assisted p-InP photocathode using, for the first time, the mechanism of TE photoemission; "Transferred Electron Photoemission from InP," R. L. Bell, L. W. James, and R. L. Moon, 25 Appl. Phys. Lett. 645 (1974). TE photoemission is based on the fact that for certain III-V semiconductors, such as InP, InGaAsP alloys, and GaAs, electrons can be promoted to the upper conduction band valleys with reasonable efficiency by applying modest electric fields. Photogenerated electrons which successfully transfer to the upper valleys, or become hot gamma electrons, are then energetic enough to have a good probability of being emitted over the work function and surface energy barriers into vacuum. Following this initial result experimental high-performance TE photocathodes for the 1000 nm to 1650 nm region were extensively investigated; "Field-Assisted Semiconductor Photoemitters for the 1-2 .mu.m Range," J. S. Escher, R. L. Bell, P. E. Gregory, S. B. Hyder, T. J. Maloney, and G. A. Antypas, IEEE Trans. Elec. Dev. ED-27, 1244 (1980).
In TE photocathodes, electrons are further elevated, or promoted, from the lowest energy states of the gamma valley of the conduction and to the upper satellite valleys (L or X) of the conduction band or to higher energy levels in the gamma valley. The promotion of electrons in a TE photocathode is accomplished by introducing an electric field of 10.sup.4 V/cm, or greater. (The field strength is a function of the doping of and the electrical bias on the semiconductor.) Because TE photocathodes rely on upper satellite valley transport almost exclusively, they are able to more readily overcome a higher threshold to escaping electrons. (This threshold is also called the "vacuum energy level.")
Various possible semiconductor materials have different bandgaps, i.e., the energy difference between their valence bands and conduction bands. On one hand, it may be desirable to choose a material with a larger bandgap, because the higher an energy an electron will jump to, the better is its probability of escaping into the vacuum. But on the other hand, large bandgap semiconductor materials require photons that have sufficient energy to cause the jump from the valence band to the now higher conduction band. The incoming photons must typically be shorter than 1000 nm in wavelength. Therefore, better electron emission comes at a cost of more limited photon wavelength sensitivity. The compromise that is often reached in NEA photocathodes is one where sensitivity to longer wavelength photons (e.g., infra-red) is achieved, at the cost of putting the sole transporting conduction band valley (e.g., gamma valley) just barely above the vacuum energy level. Because electron energy levels are so near the vacuum energy levels in NEA photocathodes, the escape probabilities of the electrons are significantly altered by small changes in the "work function" or surface barrier of the material at the photocathode to vacuum interface.
To escape the surface of a photocathode into a vacuum an electron must be sufficiently energetic to overcome the vacuum energy level. In an NEA photocathode the effective electron affinity for electrons in the gamma valley of the conduction band in the bulk material is determined by the work function at the semiconductor surface and the band binding of the semiconductor. Since the band gap region is typically no more than 100 .ANG. wide, the electrons in the gamma valley can transport across the region as hot electrons with little or no loss in energy. Thus if the band gap is larger than the work function at the semiconductor surface, the electrons have a greater probability of reaching the surface with energy sufficient to overcome the work function and thus escape into the vacuum. Low work function metals and activation layers that reduce work function have therefore been preferred in photocathode use. (See, e.g., Bell, U.S. Pat. No. 3,644,770.)
In a TE photocathode, a bias is applied to develop an electric field in the semiconductor which, by the Transferred Electron Effect, promotes the electrons to higher energy levels as they are transported through the depletion region created by the bias. The energy imparted to the electrons by the electric field allows the electrons to have an energy that, as above, is greater than the work function and thus sufficient to see that the electrons escape into the vacuum. As is described by Bell '143, a simple Schottky barrier can be implemented between the semiconductor and the activation layer, using silver to allow the application of a bias voltage. The Schottky barrier height between the semiconductor and metal needs to be sufficient to prevent appreciable hole current from flowing from the metal into the semiconductor. A large hole current would prevent application of sufficient bias to the semiconductor due to IR drops in the thin metal layer, in addition to introducing noise via the electron/hole pair creation associated with the hole current. In the prior art, the metal is uniformly applied over the whole electron emitting surface of the photocathode to allow application of a uniform bias to the semiconductor and to provide a return path for electrons that do not escape into the vacuum. Any such metal layer, however, will block some electrons from escaping because the electrons must first pass through the metal, and the electrons that are blocked add to the return current. The metal of choice has been silver, because of its ability to obtain a low work function surface by applying an activation layer of cesium and oxygen to the silver surface and its high conductivity. (Such activation lowers the work function of the metal to about 1.0 eV using Ag as the metal.) In a TE photocathode, use of silver is described by Bell '143 as his preferred embodiment.
Some TE semiconductor photocathodes are constructed of a semiconductor photon absorbing layer, a separate semiconductor electron emitting layer, with a heterojunction being formed between the two layers. In other TE semiconductor photocathodes a single semiconductor layer is used both as the photon absorbing layer and as the electron emitting layer. In either case, as is well known, the dark current for the photocathode, i.e., the current that flows in the absence of light photons, will be minimized if the proton absorbing layer is constructed of P-type material.
If non-escaping electrons were allowed to collect at a point on the surface, a charge sufficient to "bias off" the surface in the vicinity of the excess electrons would occur, and no electrons would escape thereafter. A metallization layer serves to provide a return path for these surface electrons in addition to providing a way to uniformly bias the photocathode allowing an efficient transfer of electrons from the gamma to the upper satellite valleys of the conduction band. A tradeoff must be made, however, in the metallization layer so that it is thick enough to be sufficiently conductive, given the operating conditions of the device, and yet thin enough to not present too great an obstacle to electron emission. Silver is, in general, a very "transparent" material to escaping electrons compared to other metals, but when deposited on a semiconductor surface, silver tends to clump and form islands that can only be overcome by applying a thicker layer. The advantage of silver as a high electron transparency medium is therefore lost. The net result is such a thick layer of silver must be applied that as much as 90% of the electrons produced for emission collide with the silver's atomic structure and are thereby too degraded in energy to escape. Again, those electrons not escaping must be collected and conducted away from the photocathode surface.
Another problem with prior art TE photocathodes occurs when a large flux of photons incident on a small region of a TE photocathode creates a large population of promoted electrons. While it is generally desirable to have the thinnest possible metallization layer, a very thin metallization layer exhibits relatively large resistance, which, in turn, causes the well known problem of "blooming," i.e., although the large flux of input photons is confined to a small region, a much larger region is affected. While blooming is usually understood to be the growth of a white spot on a phosphor screen, blooming in a TE photocathode causes just the opposite effect: a dark spot on a phosphor display screen will grow as the photocathode is biased-off by the large population of electrons at the photocathode surface. Since more than half of the electrons produced for emission can wind up having to be returned by the metallization layer, a large IR (current x resistance) drop will form between the spot and a device's contact pad, i.e., the point on the periphery of the photocathode where the bias voltage is applied. Prior art devices exhibit a congestion of electrons on the return path, and electrons accumulate and involve a much larger area than was initially involved. In the worst case, this congestion can bloom over the entire photocathode surface and the accumulated charges will bias-off the device completely. This phenomenon is also known in the art as "photoresponse loss."
Another practical disadvantage of the prior art is the difficulty in forming a reliable mechanical contact in a tube assembly to the contact pad and extremely thin Schottky barrier, which is required for efficient electron transmission. The electrical contact at the contact pad is likely to be intermittent if the thin metal layer is directly penetrated by the contact probe. Penetration of the metal layer is also likely to result in high field regions in the contact area resulting in unacceptably high leakage currents which will effectively shunt the Schottky barrier.
It has been determined that aluminum has very favorable heat clean thermal stability characteristics making it an excellent choice for a relatively thick contact pad applied directly to the semiconductor surface, since the post heat clean leakage currents remain low for the resulting Schottky barrier. The inventors have experimented and researched several other metals and have not found any alternatives which will survive heat clean, while maintaining a good Schottky barrier. Moreover, aluminum's ability to survive heat clean allows the creation of a grid structure, described below, by photolithography directly on the semiconductor surface. Previous art required the evaporation of thick contact pads after the heat clean step had been completed. This introduced the added complexity of thick UHV metal depositions accurately positioned on the photocathode surface. The unusual properties of aluminum include the ability to maintain a Schottky barrier on InP even after the thermal cycle associated with a heat clean, and the ability to survive the chemical processing associated with the final chemical clean (which is required prior to heat clean of the photocathode).
Because aluminum exhibits excellent thermal stability properties, it may be patterned using photolithography techniques into a grid structure prior to final chemical and heat clean of the semiconductor surface prior to activation of the photocathode. The grid structure could then simultaneously contain photoresponse losses and improve quantum efficiency.